In recent years, in ink-jet recording systems, image quality has increasingly been improved so as to approach the conventional silver salt photographic images. As a means to achieve conventional photographic image quality utilizing ink-jet recording, employed recording sheets are those which have been under rapid technical improvements. For example, recording sheets have been developed which comprise a highly smoothed substrate having thereon a porous layer comprising minute ink absorptive voids. Since said sheet exhibits high ink absorbability as well as excellent drying properties, its use has become one of the methods to generate images which are most similar to said photographic quality.
Said porous ink layer is comprised mainly of hydrophilic binders as well as fine particles. Known as such fine particles are fine inorganic or organic particles, however, fine inorganic particles with higher gloss, capable of decreasing the particle size, are generally employed.
On the other hand, regarding said substrates, known as ink-jet recording sheets are water absorptive substrates such as papers, as well as non-water absorptive substrates such as polyester films and resin coated papers. The former exhibits an advantage in relatively high ink absorbability due to its capability of absorbing ink. However, contrary to said advantage, it exhibits disadvantages as described below. Problems occur in which creases (also called cockling) after printing tend to form due to the water absorbability of said substrate. As a result, it is difficult to produce high quality prints, and the print surface tends to rub the head due to said creasing during printing.
On the contrary, the use of said non-water absorptive substrates exhibits advantages such that none of the problems described above occur and high quality prints are produced. However, the ink absorption amount is limited.
Generally, various characteristics are required for said ink absorptive layer. As a result, in order to improve said various characteristics, the use of each additive, described below, is proposed:    1: fine stable particles to result in voids of no more than approximately 0.1 μm to achieve high color forming capability as well as high gloss    2: low swellable hydrophilic binders to increase fine particle holding force as well as to minimize a decrease in the ink absorption rate    3: cross-linking agents of hydrophilic binders to enhance the ink absorption rate as well as the waterfastness of coated layers    4: surface active agents and hydrophilic polymers distributed over the surface to achieve an optimal dot diameter    5: cationic fixing agents to minimize bleeding of dyes as well as to improve the waterfastness of said dyes    6: anti-discoloring agents to minimize the discoloration of dye images due to light as well as oxidizing gases    7: optical brightening agents as well as color control agents (such as reddening agents and bluing agents) to improve white backgrounds    8: matting agents as well as slipping agents to control the slipping properties of the surface    9: various types of oil components, latex particles and water-soluble plasticizers to provide desired flexibility to coated layers    10: various inorganic salts (multivalent metal salts) to minimize the bleeding of dyes as well as to enhance the waterfastness and weather fastness of dyes    11: acids and alkalis to adjust the pH of porous layers.
However, when additives, which are employed to achieve the various purposes as described above, are added to an ink receptive layer forming coating composition, many additives are frequently subjected to various limitations from the viewpoint of stabilizing the production processes.
Production beyond said limitations may cause problems which include: for example,    A: When coagulation occurs among fine particles or among additives, or when phase separation occurs in a coating composition, problems occur which make it difficult to achieve stable uniform coating; gloss decreases to result in a matt surface; and production efficiency markedly decreases due to a decrease in the pot life of the coating composition.    B: When a prepared coating composition is kept standing over an extended period of time, said composition may markedly increase its viscosity to result in gelling or on the contrary, it may decrease its viscosity so that it tends to flow too easily on the substrate. As a result, it becomes difficult to achieve stable coating, making it difficult to obtain a uniform coating.    C: When a porous layer is coated and subsequently dried, surface cracking tends to occur.    D: The void ratio in the porous layer decreases.
Problems, which relate to items A and B, frequently occur mainly due to electrical and mutual interaction of additives. For example, cationic fixing agents may play a major role while reacting with raw materials having an anionic group so as to result in various problems.
On the other hand, problems, which relate to items C and D, occur mainly when a non-water absorptive substrate is employed as a substrate. Namely, in the case of an ink-jet recording sheet which comprises a non-water absorptive substrate having thereon a porous layer comprised of voids, it is required that during ink-jet recording, all the ink is temporarily held in the void layer as the ink absorptive layer. In order to achieve this, it is required that said ink-jet recording sheet have a large void volume. As a result, it is required that a thick-coated layer having a high void ratio be formed.
When said porous layer is applied onto said non-ink absorptive layer, it is required that the dried layer thickness is generally at least 25 μm, and is more preferably from 30 to 50 μm. When such a relatively thick porous layer is applied onto a substrate, problems occur during the production period, in which cracking tends to occur during drying due to the fact that said porous layer is stiff, or the void ratio decreases. Particularly, when various additives, to achieve various functions, are added to the porous layer forming coating composition, said adverse problems are more exhibited.
It is assumed that cracking of said porous layer is due to the contraction of the coating during drying and depends on various factors. As one of these factors, it is assumed that the presence of additives, which decrease the holding ability of fine particles in the hydrophilic binder, tends to result in said cracking.
Further, the void ratio tends to increase through the interaction between fine particles or between hydrophilic binders. However, said interaction may decrease upon employing a certain type of additive, whereby said void ratio occasionally decreases. Namely, each of fine particles tends to be subjected to closest packing.
Generally, at present, the usable ratio of hydrophilic binders to fine particles is limited to no more than 1/2 in terms of weight ratio, and specifically no more than 1/3, so that the ink absorbability does not decrease, while voids, which generate porosity, are filled. As a result, the ability of said hydrophilic binders to protect fine colloidal particle tends to be affected by a small amount of additives.
During the drying process of a porous layer, said porous layer is formed in such a manner that while a small amount of binder is covering the surface of fine particles, said binder, which is entwined with each other, protect colloidal fine particles. However, during said process, if additives are present, said binders are not sufficiently entwined with each other. Therefore, it is assumed that the resulting layer strength decreases and cracking tends to occur during the period in which the layer is subjected to contraction during the drying process, especially during the period in which drying is almost completed.
Usually, ink-jet recording sheets are produced by applying a porous layer forming coating composition onto a continuously conveyed substrate, and subsequently winding the resulting coating into a roll after drying. Thereafter, a coating composition comprised of specified additives is applied onto the surface of said coating. This is known as a so-called overcoat applying method (hereinafter referred to as an overcoating method). However, since in most of said overcoating methods, coating is carried out while divided into at least two time frames, problems occur resulting in markedly increased production cost. Further, other problems tend to occur in which quality consistency is degraded due to the temperature history as well as time fluctuation during temporary storage after the formation of said porous layer and in addition, non-uniform coating tends to occur during said overcoating.
Generally, the layer state just after coating and drying is often different from its state after storage, depending on the existing state of the hydrophilic binders, the ongoing crystallization, and the re-orientation of the distribution of additives in the layer.
It is assumed that immediately after coating and drying, said hydrophilic binders are distributed nearly equal to their solution state. However, when they are stored and subjected to an equilibrium state, the swellability of the resulting layer tends to decrease due to the interaction (being the cross-linking reaction) between the fine particles and the additives. Specifically, polyvinyl alcohol, which is most useful as a hydrophilic binder, often results in variation of water absorbability of the overcoated composition as well as swellability with respect to the hydrophilic binders.
Further, surface active agents, hydrophilic binders, and other diffusible additives on the surface of said porous layer tend to be gradually modified after being coated, so as to adversely affect the wettability of said overcoating composition. As a result, during coating of said overcoating composition, non-uniform coating tends to occur.
Still further, variation of said porous layer during its storage period results in difference in the distribution of overcoated additives in said layer, as well as differences in their effects. As a result, said variation may become a fluctuation factor to result in the resulting product quality.
A method, in which an additive containing coating composition is applied onto said porous layer so as to supply additives to said porous layer, is known as a so-called impregnation method when ink-jet recording sheets are produced. However, no method at all is known in which, just after forming said porous layer, the production is carried out utilizing an on-line means.
Japanese Patent Publication Open to Public Inspection No. 11-115308 describes a method in which a coating composition, comprising fine inorganic particles as well as a water-soluble resin, is applied onto a substrate and at the same time of coating or before the coated layer reaches falling drying rate, said coated layer is hardened by providing a solution comprising a cross-linking agent of said water-soluble resin. “Before the coated layer reaches falling drying rate”, as described herein, refers to a constant drying rate period, as described in said patent, and the drying period in which the content of solvents in the coating decreases in proportion to time when fixed drying conditions are maintained.
However, investigations conducted by the inventors of the present invention revealed that problems generally occurred at high speed coating as described below. When said additives are overcoated during the constant drying rate period, the solvents of the preceding coating composition are not sufficiently evaporated. As a result, cracking occurs in the final period of drying, or at the time when the overcoating is carried out, the surface of the applied overcoating composition is subjected to a drying airflow as well as mechanical vibration. As a result, airflow unevenness, that is, coating unevenness due to variation of airflow which hits said surface, as well as transporting unevenness, tends to occur.
Japanese Patent Publication Open to Public Inspection No. 8-34160 describes a method in which a solution, comprising a silane coupling agent having a quaternary ammonium salt group, is overcoated onto the surface of a layer comprised of fine inorganic particles and a water-soluble resin. However, in the method claimed in said patent, a solution comprising an additive is overcoated onto the surface of a porous layer which has been obtained by drying, and further said patent describes neither the formation of a porous layer targeted by the present invention nor the coating of a solution utilizing an on-line means. Said patent depends on a conventional and common method, whereby problems with said coating unevenness and insufficient quality stability are not overcome.
Accordingly, a first object of the present invention is to provide an ink-jet recording sheet which comprises a substrate having thereon a porous layer having a high void ratio, which minimizes the formation of cracking during production, even though various additives are incorporated in said porous layer comprised of hydrophilic binders as well as fine particles. A second object of the present invention is to provide an ink-jet recording sheet which exhibits high quality without a marked increase in production cost when a solution comprising additives is overcoated after applying a coating composition forming a porous layer onto a substrate. A third object of the present invention is to provide a high quality ink-jet recording sheet in which the fluctuation of manufacturing quality is minimized and coating uniformity is improved.